The present invention relates to an apparatus and method for processing materials using radiant energy.
Many items are heated for processing operations. There are three main methods of heat transfer, namely conduction, convection, and radiation. For each of these methods, there are benefits and drawbacks for various materials in various situations. As a background, each of the heat transfer methods are rated on the following criteria: speed, efficiency, controllability of heat transfer rate, and controllability of spatial heat placement. In layman's terms, how fast is it heated, how cheaply is it heated, how accurately is it heated, and how well is the heat placed. The ideal heater is fast, cheap, accurate, and able to place heat accurately.
Speed (Heat Transfer Rate)
1) Conduction is heat transfer through solids. One hot object touches another and quickly transfers heat from the hot object to the cooler object. If the objects are good conductors, the heat will transfer quickly.
2) Convection is heat transfer using a fluid. The fluid properties determine the rate of heat transfer. Hot water can transfer heat quickly because it conducts well, and has a high specific heat. Hot air transfers heat quite slowly. Air is generally an insulator and has a low specific heat.
3) Radiation is heat transfer using electromagnetic energy. This energy travels at the speed of light, so it is the absolute fastest method of heat transfer. However, the target material must absorb this energy. If the material is a poor absorber, either reflecting, transmitting, or re-emiting, it will not heat up. So, the ultimate rate of heat transfer depends upon what fraction of the radiation is absorbed by the material and converted to heat. Additionally, if the radiation is absorbed at the material surface, the subsurface material will be heated through conduction from the material surface.
Efficiency (Cheap to Operate)
1) Conduction. Typically, the hot object has substantial mass in order to build up a store of heat energy. This mass must be maintained at a controlled temperature, and then placed in contact with the object to be heated. The contact time is controlled to obtain the desired amount of heat transfer. The hot object will be continuously giving off heat to the surrounding environment. Insulation can be added to reduce the heat losses. As long as the hot object is hot, there is a heat loss that reduces efficiency.
2) Convection. Typically, the working fluid is heated and contained within an insulated vessel. The vessel and working fluid have mass that provides heat storage. Similar to conduction, the convection system will be continuously losing heat to the environment. Since convection requires a vessel, the volume to be insulated is larger than an equivalent conduction system. With air convection, there can be significant losses of heat as air escapes and products enter the vessel.
3) Radiation. There are several methods of generating radiation. Several have the ability to be turned on and off extremely quickly. The types that can be turned off are more efficient, since no energy is lost while they are turned off. Some radiation generators are inefficient converters, only converting a small percentage of input energy into the desired output energy. The rest of the energy may be in an undesirable form such as waste heat. This may require a cooling system that lowers the overall efficiency. If some waste heat can be used by another part of the process, then, it is not wasted.
Accuracy (of Heating)
1) Conduction. The amount of heat transferred with a conduction system is controlled primarily by two factors. The temperature of the objects, and the contact time. These items may be controlled to a high accuracy. There must be sufficient time after each cycle for the hot item to reheat and stabilize to the levels of the prior cycle. Another variable that may greatly affect the heat transfer is the quality of the contact. If the contact is poor, the amount of heat transfer could be greatly reduced.
2) Convection. The amount of heat transferred with a convection system is also primarily controlled by temperature and contact time. In the case of air heating, the flow dynamics are of significant importance to the quality of the contact between the air and the object. Since airflow is very difficult to control in all situations, the accuracy of heating with air is also very difficult to control.
3) Radiation. Several methods of generating radiation are highly controllable. A laser beam may be pulsed or otherwise modulated to achieve an exacting output of energy. The target material absorbs this energy. If the absorption spectrum of the material is consistent, then, the accuracy of heating will be highly accurate.
Accuracy (of Placement)
1) Conduction. The heat is transferred wherever contact is made. This can be highly controlled by shaping the hot item. (Branding iron). As the contact is maintained, the heat will conduct to the cooler areas of the target material. Since conduction requires physical contact, it can be extremely difficult to construct a mechanism to heat randomly located or moving items.
2) Convection. The heat is transferred by fluid contact. The fluid flow can be very difficult to predict and control. The shape of the target object directly affects the locations of high heat transfer. This makes convection the least accurate for placement.
3) Radiation. Radiation generally travels in perfectly straight lines. As it encounters materials, it may be reflected, diffused, focused, or absorbed. In the case of a laser beam, the radiant energy may be controlled with extreme precision.
When reviewing the previous criteria, radiation has the potential to be the fastest, the most efficient, and the most accurate for heat transfer and heat placement. Because of all these benefits, there is a great need to develop a low cost system that can meet all these criteria. The primary obstacle to using radiation is the absorption spectrum of the target material not matching the spectrum of energy being generated. A second obstacle is the added cost of a control system required to make this type of system perform well. The control costs are primarily associated with the mechanisms needed for accuracy of heating and accuracy of placement.
The present invention relates to using radiant heat to process various materials in various applications. The following examples, while instructive of the concepts and implementations involved, should not be viewed as exclusive.